Optical fiber and optical transmission system including the same

ABSTRACT

The present invention relates to an optical fiber which enables favorable optical communications in 1.3-μm and 1.55-μm wavelength bands, and an optical transmission system including the same. The optical fiber according to the present invention has only one zero-dispersion wavelength within a wavelength range of 1.20 μm to 1.60 μm, the zero-dispersion wavelength existing within a wavelength range of 1.37 μm to 1.50 μm, and has a positive dispersion slope at the zero-dispersion wavelength, thereby enabling favorable optical communications utilizing each signal light in the 1.3-μm and 1.55-μm wavelength bands sandwiching the zero-dispersion wavelength.

RELATED APPLICATIONS

[0001] This is a Continuation-In-Part application of InternationalPatent Application Ser. No. PCT/JP99/06611 filed on Nov. 26, 1999, nowpending.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an optical fiber applicable to atransmission line in optical communications, and an optical transmissionsystem including this optical fiber.

[0004] 2. Related Background Art

[0005] Conventionally, as a transmission line in optical communications,standard single-mode optical fibers having a zero-dispersion wavelengthin a 1.3-μm wavelength band (1280 nm to 1320 nm) have mainly beenutilized. The transmission loss resulting from the main material(silica) of such an optical fiber has been known to become the lowest ina 1.55-μwavelength band (1530 nm to 1565 nm). In addition, optical fiberamplifiers using an Er-doped optical fiber can amplify light in the1.55-μm wavelength band at a high efficiency. For such a reason,dispersion-shifted optical fibers designed so as to have azero-dispersion wavelength in the 1.55-μm wavelength band are applied totransmission lines in wavelength division multiplexing (WDM)communications for transmitting a plurality of wavelengths of signallight. As for a light source for sending out signal light, devicetechnologies for enabling light in the 1.3-μm wavelength band and lightin the 1.55-μm wavelength band to be outputted have conventionally beenestablished.

SUMMARY OF THE INVENTION

[0006] The inventors have studied the prior art mentioned above and, asa result, found problems as follows. Namely, in the case where light inthe 1.3-μm wavelength band is transmitted while a dispersion-shiftedoptical fiber having a zero-dispersion wavelength in the 1.55-μmwavelength band is used as an optical transmission line, the absolutevalue of dispersion becomes so large that WDM communications cannot becarried out in a wide band. Also, when signal light in the 1.55-μmwavelength band is transmitted through such a dispersion-shifted opticalfiber, the absolute value of dispersion becomes so small that four-wavemixing, which is one of nonlinear optical phenomena, is likely to occur.In the case where light in the 1.3-μm wavelength band is transmittedwhile a standard single-mode optical fiber having a zero-dispersionwavelength in the 1.3-μm wavelength band is used as an opticaltransmission line, on the other hand, the absolute value of dispersionbecomes so small that four-wave mixing, which is one of nonlinearoptical phenomena, is likely to occur. Also, when signal light in the1.55-μm wavelength band is transmitted through such a single-modeoptical fiber, the absolute value of dispersion becomes so large thatWDM communications cannot be carried out in a wide band.

[0007] For this matter, attempts have been made to develop opticalfibers for suppressing the occurrence of dispersion over a widewavelength band (see, for example, K. Okamoto et al., “Zero total insingle-mode optical fibers over an extended spectral range,” RadioScience, Volume 17, Number 1, pages 31-36, January-February 1982). Forexample, an optical fiber having a low dispersion value over a widewavelength band has been proposed by yielding a large relativerefractive index difference of 2.4% between its cladding region and coreregion and a small diameter of 3.5 μm in the core region. However, it isdifficult to make such an optical fiber having a very large relativerefractive index difference between the cladding region and core region,and its transmission loss is large. In an optical fiber whose coreregion has a smaller diameter, on the other hand, the effective areabecomes smaller, and nonlinear optical phenomena are likely to occur.

[0008] In order to overcome problems such as those mentioned above, itis an object of the present invention to provide an optical fiber whichenables efficient transmission of both of signal light in the 1.3-μmwavelength band and signal light in the 1.55-μm wavelength band, and anoptical transmission system including the same.

[0009] The optical fiber according to the present invention is anoptical fiber which enables efficient transmission of both of signallight in the 1.3-μm wavelength band and signal light in the 1.55-μmwavelength band, the optical fiber having only one zero-dispersionwavelength within a wavelength range of 1.20 μm to 1.60 μm and having apositive dispersion slope at the zero-dispersion wavelength. Here, thiszero-dispersion wavelength lies within a wavelength range of 1.37 μm to1.50 μm sandwiched between the 1.3-μm wavelength band and the 1.55-μmwavelength band. Also, the above-mentioned dispersion slope preferablyhas an absolute value of 0.10 ps/nm²/km or less at the above-mentionedzero-dispersion wavelength (preferably 0.06 ps/nm²/km or less at awavelength of 1.55 μm), and monotonously changes (e.g. ,monotonouslyincreases) at least in a wavelength range of 1.30 μm to 1.55 μm.

[0010] Thus, since this optical fiber has a zero-dispersion wavelengthwithin the wavelength range of 1.37 μm to 1.50 μm including a wavelengthof 1.38 μm at which an increase in transmission loss caused by OHabsorption is seen, dispersion occurs to a certain extent in thevicinity of the 1.3-μm wavelength band and in the vicinity of the1.55-μm wavelength band. As a consequence, the optical fiber comprises astructure in which four-wave mixing is hard to occur even when thesignal light in the 1.3-μm wavelength band and the signal light in the1.55-μm wavelength band propagate therethrough.

[0011] In the case where a thulium-doped fiber amplifier having anamplification band in a 1.47-μm wavelength band is utilized, thezero-dispersion wavelength is more preferably set within a wavelengthrange of 1.37 μm to 1.43 μm. It is because of the fact that thetransmission band can further be widened if the zero-dispersionwavelength is aligned with a skirt of the OH absorption peak (1.38 μm).In the case where the above-mentioned OH absorption peak is kept low bydehydration processing or the like, so as to utilize the wavelength bandincluding the wavelength of 1.38 μm as its signal light wavelength band,on the other hand, the zero-dispersion wavelength may be set within awavelength range of longer than 1.45 μm but not longer than 1.50 μm inorder to intentionally generate dispersion in the above-mentionedwavelength band.

[0012] In the optical fiber, while the dispersion slope monotonouslyincreases, the absolute value of the dispersion slope at itszero-dispersion wavelength is 0.10 ps/nm²/km or less, and the dispersionslope at a wavelength of 1.55 μm is preferably 0.06 ps/nm²/km or less,whereby the dispersion in the 1.3-μm wavelength band and the dispersionin the 1.55-μm wavelength band are homogenized. Here, each of theabsolute value of dispersion in the 1.3-μm wavelength band and theabsolute value of dispersion in the 1.55-μm wavelength band is 6ps/nm/km or more but 12 ps/nm/km or less.

[0013] As mentioned above, the optical fiber according to the presentinvention realizes efficient optical communications in both of the1.3-μm wavelength band and the 1.55-μm wavelength band. From theviewpoint of guaranteeing a single mode, the case where the cutoffwavelength is 1.3 μm or shorter while the transmission line length isseveral hundreds of meters or less is preferable since only theground-mode light can propagate in each of the 1.3-μm wavelength bandand the 1.55-μm wavelength band. Also, in view of the dependence ofcutoff wavelength on distance, no practical problem occurs in opticaltransmission over a relatively long distance (a transmission line lengthof several kilometers or less) even if the cutoff wavelength is 1.45 μmor shorter (in the case where it is longer than the signal lightwavelength). From the viewpoint of reducing the bending loss, on theother hand, there are cases where the bending loss increases remarkablywhen the cutoff wavelength is shorter than 1.0 μm. As a consequence, thecutoff wavelength is preferably 1.05 μm or more, more preferably 1.30 μmor more.

[0014] Further, in order to enable efficient optical transmission in the1.3-μm wavelength band and 1.55-μm wavelength band, the optical fiberaccording to the present invention has a bending loss which becomes 0.5dB or less, preferably 0.06 dB or less, per turn when wound at adiameter of 32 mm at a wavelength of 1.55 μm, and has an effective areaA_(eff) which becomes 45 μm² or more, preferably greater than 49 μm² ata wavelength of 1.55 μm. Also, the amount of increase in transmissionloss caused by OH absorption at a wavelength of 1.38 μm in the opticalfiber is 0.1 dB/km or less. In particular, if the amount of increase intransmission loss caused by OH absorption at a wavelength of 1.38 μm is0.1 dB/km or less, then a wavelength band in the vicinity of thiswavelength of 1.38 μm can be utilized for a signal light wavelengthband. In this case, in order to intentionally generate dispersion in thewavelength band in the vicinity of the wavelength of 1.38 μm (in orderto suppress four-wave mixing), the zero-dispersion wavelength may be setwithin a wavelength range of longer than 1.45 μm but not longer than1.50 μm.

[0015] Here, the effective area A_(eff) is given, as shown in JapanesePatent Application Laid-Open No. HEI 8-248251 (EP 0 724 171 A2), by thefollowing expression (1): $\begin{matrix}{A_{eff} = {2{{\pi \left( {\int_{0}^{\infty}{E^{2}r\quad {r}}} \right)}^{2}/\left( {\int_{0}^{\infty}{E^{4}r\quad {r}}} \right)}}} & (1)\end{matrix}$

[0016] where E is the electric field accompanying the propagated light,and r is the radial distance from the core center.

[0017] The optical fiber according to the present invention has arefractive index profile in which the maximum and minimum values ofrelative refractive index difference with reference to the refractiveindex of pure silica (silica which is not intentionally doped withimpurities) are 1% or less and −0.5% or more, respectively. In such arefractive index profile, the relative refractive index difference of ahigh refractive index region doped with Ge element, for example, withrespect to pure silica is 1% or less, whereas the relative refractiveindex difference of a low refractive index region doped with F element,for example, with respect to pure silica is −0.5% or more, whereby itsmanufacture (refractive index control by doping with impurities) iseasy, and the transmission loss can be lowered. Here, the minimum valueof relative refractive index difference with reference to the refractiveindex of pure silica is preferably −0.2% or more, more preferablygreater than −0.15% from the viewpoint of facilitating the manufactureof the optical fiber.

[0018] The optical fiber having various characteristics such as thosementioned above can be realized by various configurations. Namely, afirst configuration of the optical fiber can be realized by a structurecomprising a core region which extends along a predetermined axis andhas a predetermined refractive index, and a cladding region provided onthe outer periphery of the core region. The optical fiber of the firstconfiguration may further comprise a depressed cladding structure. Thedepressed cladding structure is realized when the above-mentionedcladding region is constituted by an inner cladding, provided on theouter periphery of the core region, having a lower refractive index thanthe core region; and an outer cladding, provided on the outer peripheryof the inner cladding, having a refractive index higher than that of theinner cladding but lower than that of the core region.

[0019] As with the first configuration, a second configuration of theoptical fiber comprises a core region and a cladding region provided onthe outer periphery of the core region. However, the core region isconstituted by a first core having a predetermined refractive index; anda second core, provided on the outer periphery of the first core, havinga lower refractive index than the first core. In the case where theoptical fiber of the second configuration comprises a depressed claddingstructure, the cladding region is constituted by an inner cladding, incontact with the outer periphery of the second core, having a lowerrefractive index than the second core; and an outer cladding, providedon the outer periphery of the inner cladding, having a refractive indexhigher than that of the inner cladding but lower than that of the secondcore.

[0020] As with the first configuration, a third configuration of theoptical fiber comprises a core region extending along a predeterminedaxis and a cladding region provided on the outer periphery of the coreregion. In particular, the core region comprises a first core having apredetermined refractive index; a second core, provided on the outerperiphery of the first core, having a lower refractive index than thefirst core; and a third core, provided on the outer periphery of thesecond core, having a higher refractive index than the second, core. Inthe case where the optical fiber of the third configuration comprises adepressed cladding structure, the cladding region is constituted by aninner cladding, in contact with the outer periphery of the third core,having a lower refractive index than the third core; and an outercladding, provided on the outer periphery of the inner cladding, havinga refractive index higher than that of the inner cladding but lower thanthat of the third core.

[0021] When the third configuration mentioned above is employed, itbecomes easier to obtain an optical fiber having a low dispersion slopeof 0.06 ps/nm²/km or less at a wavelength of 1.55 μm in particular.

[0022] Further, a fourth configuration of the optical fiber alsocomprises a core region extending along a predetermined axis and acladding region provided on the outer periphery of the core region. Inparticular, the core region comprises a first core having apredetermined refractive index; a second core, provided on the outerperiphery of the first core, having a higher refractive index than thefirst core. In the case where the optical fiber of the fourthconfiguration comprises a depressed cladding structure, the claddingregion is constituted by an inner cladding, in contact with the outerperiphery of the second-core, having a lower refractive index than thesecond core; and an outer cladding, provided on the outer periphery ofthe inner cladding, having a refractive index higher than that of theinner cladding but lower than that of the second core.

[0023] A fifth configuration of the optical fiber comprises a coreregion extending along a predetermined axis and a cladding regionprovided on the outer periphery of the core region. In particular, thecore region comprises a first core having a predetermined refractiveindex; a second core, provided on the outer periphery of the first core,having a higher refractive index than the first core; a third core,provided on the outer periphery of the second core, having a lowerrefractive index than the second core; and a fourth core, provided onthe outer periphery of the third core, having a higher refractive indexthan the third core. In this fifth mode of optical fiber, the claddingregion has a lower refractive index than the fourth core.

[0024] The optical transmission system according to the presentinvention is realized by the optical fiber having such a configurationas those mentioned above. Specifically, the optical transmission systemaccording to the present invention comprises, at least, a firsttransmitter for outputting first light in the 1.3-μm wavelength band, asecond transmitter for outputting second light in the 1.55-μm wavelengthband, a multiplexer for multiplexing the first light outputted from thefirst transmitter and the second light outputted from the secondtransmitter, and an optical fiber comprising a configuration mentionedabove and having one end thereof optically connected to the multiplexer.As a result of this structure, the optical fiber transmits each of thefirst light and second light multiplexed by the multiplexer. Accordingto the optical transmission system having such a structure, the firstlight in the 1.3-μm wavelength band outputted from the first transmitteris made incident on the above-mentioned optical fiber by way of themultiplexer and propagates through the optical fiber toward a receivingsystem. On the other hand, the second light in the 1.55-μm wavelengthband outputted from the second transmitter is made incident on theoptical fiber by way of the multiplexer and propagates through theoptical fiber toward the receiving system. Also, as mentioned above, theoptical fiber applied to the optical transmission line comprises astructure enabling efficient optical communications in each of the1.3-μm wavelength band and 1.55-μm wavelength band, whereby the opticaltransmission system enables large-capacity communications when theoptical fiber having such a special structure is employed therein.

[0025] The present invention will be more fully understood from thedetailed description given hereinbelow and the accompanying drawings,which are given by way of illustration only and are not to be consideredas limiting the present invention.

[0026] Further scope of applicability of the present invention willbecome apparent from the detailed description given hereinafter.However, it should be understood that the detailed description andspecific examples, while indicating preferred embodiments of theinvention, are given by way of illustration only, since various changesand modifications within the spirit and scope of the invention will beapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1A is a graph showing a transmission loss characteristic ofan optical fiber according to the present invention with respect towavelength, whereas FIG. 1B is a graph showing a dispersioncharacteristic of the optical fiber according to the present inventionwith respect to wavelength;

[0028]FIG. 2A is a view showing a cross-sectional structure of first andthirteenth embodiments of the optical fiber according to the presentinvention, whereas FIG. 2B is a refractive index profile of the opticalfiber according to the first embodiment shown in FIG. 2A;

[0029]FIG. 3 is a refractive index profile of an optical fiber accordingto a second embodiment;

[0030]FIG. 4 is a refractive index profile of optical fibers accordingto third, fifteenth, and seventeenth embodiments;

[0031]FIG. 5 is a refractive index profile of an optical fiber accordingto a fourth embodiment;

[0032]FIG. 6 is a refractive index profile of optical fibers accordingto fifth, sixteenth, eighteenth, nineteenth, and twenty-firstembodiments;

[0033]FIG. 7 is a refractive index profile of optical fibers accordingto sixth, twentieth, and twenty-second embodiments;

[0034]FIG. 8 is a refractive index profile of optical fibers accordingto seventh and eighth embodiments;

[0035]FIG. 9 is a refractive index profile of optical fibers accordingto ninth and tenth embodiments;

[0036]FIG. 10 is a refractive index profile of optical fibers accordingto eleventh and twelfth embodiments;

[0037]FIG. 11 is a table listing various characteristics of the opticalfibers according to the first to thirteenth embodiments having variousrefractive index profiles as shown in FIGS. 2D and 3 to 10;

[0038]FIG. 12 is a table listing various characteristics of the opticalfibers according to the fourteenth to twenty-second embodiments;

[0039]FIG. 13 is a graph showing a dispersion characteristic of theoptical fiber according to the first embodiment with respect towavelength;

[0040]FIG. 14 is a graph showing a transmission loss characteristic withrespect to wavelength of an optical fiber according to the firstembodiment in which dehydration processing has been insufficient;

[0041]FIG. 15 is a graph showing a transmission loss characteristic withrespect to wavelength of optical fibers according to the first andthirteenth embodiments in which dehydration processing has been carriedout sufficiently;

[0042]FIG. 16 is a graph showing a transmission loss characteristic withrespect to wavelength of an optical fiber according to the thirteenthembodiment in which dehydration processing has been insufficient;

[0043]FIG. 17A is a graph showing relationships between effective areaA_(eff) and dispersion slope at a wavelength of 1.55 μm mainlyconcerning the eighteenth to twenty-second embodiments, whereas FIG. 17Bis a graph showing relationships between cutoff wavelength λc andbending loss per turn when bent at a diameter of 32 mm at a wavelengthof 1.55 μm concerning main embodiments; and

[0044]FIG. 18A is a view showing a schematic configuration of theoptical transmission system according to the present invention, whereasFIG. 18B is a view showing a modified example of the opticaltransmission system according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] In the following, embodiments of the optical fiber and opticaltransmission system according to the present invention will be explainedwith reference to FIGS. 1A to 2B, 3 to 16, and 17A to 18B. Among thedrawings, constituents identical to each other will be referred to withnumerals or letters identical to each other without repeating theiroverlapping explanations.

[0046] First, FIG. 1A is a graph showing a transmission losscharacteristic of an optical fiber according to the present inventionwith respect to wavelength, whereas FIG. 1B is a graph showing adispersion characteristic of the optical fiber according to the presentinvention with respect to wavelength.

[0047] The optical fiber according to the present invention has only onezero-dispersion wavelength within a wavelength range of 1.20 μm to 1.60μm, whereas this zero-dispersion wavelength exists within a wavelengthrange of 1.37 μm to 1.50 μm. Since a transmission loss due to OHabsorption occurs near a wavelength of 1.38 μm as shown in the graph ofFIG. 1A (see, for example, KAZUHIRO NOGUCHI et al., “Loss Increase forOptical Fibers Exposed to Hydrogen Atmosphere,” JOURNAL OF LIGHTWAVETECHNOLOGY, VOL. LT-3, NO. 2, APRIL 1985), it is not always favorable toapply signal light in the vicinity of this wavelength to opticalcommunications. Therefore, as shown in FIG. 1B, the zero-dispersionwavelength in the optical fiber according to the present invention isset within a wavelength range of 1.37 μm to 1.43 μm including thewavelength of 1.38 μm at which the transmission loss due to OHabsorption occurs, but is kept from being set in the vicinity of the1.3-μm wavelength band and 1.5-μm wavelength band sandwiching thiswavelength band. When the wavelength band including the wavelength of1.38 μm is utilized as a signal light wavelength band, thezero-dispersion wavelength may be set within a range of longer than 1.45μm but not longer than 1.50 μm. Thus, the 1.3-μm wavelength band and1.55-μm wavelength band deviated from a predetermined wavelength bandincluding the zero-dispersion wavelength are utilized as a signalwavelength band in the optical fiber according to the present invention,so that dispersion is intentionally generated in these wavelength bands,while the occurrence of four-wave mixing is effectively suppressed. Whenthe zero-dispersion wavelength is set within the range of 1.37 μm to1.43 μm as mentioned above, the transmission band can further be widenedby use of a thulium-doped fiber amplifier whose amplification band liesin a 1.47-μm wavelength band. In the case where the above-mentioned OHabsorption peak is kept low by dehydration processing or the like, so asto utilize the wavelength band including the wavelength of 1.38 μm as asignal light wavelength band, on the other hand, the zero-dispersionwavelength may be set within a wavelength range of longer than 1.45 μmbut not longer than 1.50 μm in order to intentionally generatedispersion in the above-mentioned wavelength band.

[0048] Also, the optical fiber according to the present invention has adispersion with an absolute value of 0.10 ps/nm²/km or less at theabove-mentioned zero-dispersion wavelength (preferably 0.06 ps/nm²/km orless at a wavelength of 1.55 μm), thereby being able to realizehomogenization of the respective dispersions in the 1.3-μm wavelengthband and 1.55-μm wavelength band. Here, in this optical fiber, each ofthe absolute value of dispersion D_(1.3) at a wavelength of 1.3 μm andthe absolute value of dispersion D_(1.55) at a wavelength of 1.55 μm is6 ps/nm/km or more but 12 ps/nm/km or less. Even in view of the factthat a standard single-mode optical fiber having a zero-dispersionwavelength in the 1.3-μm wavelength band has a dispersion value of about17 ps/nm/km in the 1.55-μm wavelength band, the optical fiber accordingto the present invention has a sufficiently small absolute value ofdispersion (12 ps/nm/km or less) in each of the 1.3-μm wavelength bandand 1.55-μm wavelength band, thus being more suitably utilized inoptical communications. Since dispersion occurs to an appropriate extent(6 ps/nm/km or more) in these wavelength bands, on the other hand,four-wave mixing can effectively be kept from occurring.

[0049] Further, from the viewpoint of guaranteeing a single mode, theoptical fiber according to the present invention preferably has a cutoffwavelength of 1.3 μm or shorter when its transmission length is notlonger than several hundreds of meters. In this case, only ground-modelight can propagate in each of the 1.3-μm wavelength band and 1.55-μmwavelength band. Also, in view of the dependence of cutoff wavelength ondistance, the cutoff wavelength may be 1.45 μm or shorter in opticaltransmission over a relatively long distance (a transmission line lengthof several kilometers or less). In this specification, the cutoffwavelength is that of LP11 mode measured in a state where an opticalfiber having a length of 2 m is wound by only one turn at a radius of140 mm as defined in a CCITT standard. From the viewpoint of reducingthe bending loss, there are cases where the bending loss remarkablyincreases when the cutoff wavelength is shorter than 1.0 μm. Therefore,the cutoff wavelength is preferably 1.05 μm or more, more preferably1.30 μm or more.

[0050] As mentioned above, the optical fiber according to the presentinvention is a single-mode optical fiber in which a zero-dispersionwavelength is set within a wavelength range deviated from both of the1.3-μm wavelength band and 1.55-μm wavelength band, while the dispersionvalue is small in each wavelength band, whereby it is suitable as atransmission medium in an optical communication system utilizing aplurality of wavelength bands.

[0051] The optical fiber according to the present invention preferablyhas a dispersion slope monotonously changing within a wavelength rangeof 1.30 μm to 1.55 μm (monotonously increasing in the case shown in FIG.1B). This case is preferable not only in that only one zero-dispersionwavelength can be set within a wavelength range of 1.20 μm to 1.60 μm,but also in that the dispersion in each of the 1.3-μm wavelength bandand1.55-μm wavelength band would not approach zero (because nonlinearoptical phenomena are likely to occur when the dispersion approacheszero).

[0052] The optical fiber according to the present invention preferablyhas a bending loss of 0.5 dB/turn or less, more preferably 0.06 dB/turnor less at a wavelength of 1.55 μm when wound at a diameter of 32 mm. Inthis case, since the bending loss is sufficiently small, the increase inloss caused by cabling and the like can effectively be suppressed. Here,this bending loss (dB/turn) is a value obtained when the transmissionloss of light having a wavelength of 1.55 μm concerning an optical fiberwound about a mandrel having a diameter of 32 mm is converted into aloss value per turn.

[0053] In the optical fiber according to the present invention, theeffective area A_(eff) at a wavelength of 1.55 μm is preferably 45 μm²or more, more preferably greater than 49 μm². This value is on a parwith or greater than the effective area in a conventionaldispersion-shifted optical fiber having a zero-dispersion wavelength inthe 1.55-μm wavelength band, so that the optical intensity per unitcross-sectional area decreases, whereby the occurrence of nonlinearoptical phenomena such as four-wave mixing is effectively suppressed.

[0054] In the optical fiber according to the present invention, theamount of increase α in transmission loss caused by OH absorption at awavelength of 1.38 μm is preferably 0.1 dB/km or less. It is because ofthe fact that the wavelength band applicable to optical communicationsis widened thereby, so as to enable larger-capacity opticalcommunications. In the case where a wavelength band including awavelength of 1.38 μm is utilized as a signal light wavelength band, thezero-dispersion wavelength is preferably designed to lie within awavelength range of longer than 1.45 μm but not longer than 1.50 μmdeviated from the above-mentioned wavelength band, in order to suppressthe occurrence of nonlinear optical phenomena.

[0055] Preferably, the optical fiber according to the present inventionhas a refractive index profile in which the maximum and minimum valuesof relative refractive index difference with reference to the refractiveindex of pure silica (silica which is not intentionally doped withimpurities) are 1% or less and −0.5% or more, respectively. Since therelative refractive index difference of a high refractive index regiondoped with Ge element, for example, with respect to pure silica is 1% orless, whereas the relative refractive index difference of a lowrefractive index region doped with F element, for example, with respectto pure silica is −0.5% or more, an optical transmission medium which isrelatively easy to make and has a low transmission loss is obtained. Forfurther facilitating the manufacture, the minimum value of relativerefractive index difference with reference to the refractive index ofpure silica is preferably −0.2% or more, more preferably greater than−0.15%.

[0056] First to twenty-second embodiments of the optical fiber accordingto the present invention will now be explained with reference to FIGS.2A, 2B, and 3 to 10.

[0057] (First Embodiment)

[0058]FIG. 2A is a view showing a cross-sectional structure of anoptical fiber 100 according to the first embodiment, whereas FIG. 2B isa refractive index profile of the optical fiber 100 shown in FIG. 1A.The optical fiber 100 according to the first embodiment comprises a coreregion 110, with an outside diameter 2 a, extending along apredetermined axis and having a refractive index n₁; and a claddingregion 120, provided on the outer periphery of the core region 110,having a refractive index n₂ (<n₁). Here, the refractive index of thecore region 110 is higher than that of the cladding region 120. Theoutside diameter 2 a of the core region 110 is 5.2 μm, whereas therelative refractive index difference Δ₁ of the core region 110 withreference to the cladding region 120 is 0.55%. Such an optical fiber isobtained when, while silica is used as a base, the core region 110 isdoped with Ge element, for example.

[0059] The abscissa of the refractive index profile 150 shown in FIG. 2Bcorresponds to individual parts, along the line L in FIG. 2A, on a crosssection perpendicular to the center axis of the core region 110. Hence,in the refractive index profile 150 of FIG. 2B, areas 151 and 152indicate the refractive indices in individual parts on the line L of thecore region 110 and the cladding region 120, respectively.

[0060] Here, the relative refractive index difference Δ₁ of the coreregion 110 with respect to the outermost cladding region 120 is definedas follows:

Δ₁=(n ₁ −n ₂)/n ₂

[0061] where n₁ is the refractive index of the core region 110, and n₂is the refractive index of the cladding region 120. Also, in thisspecification, the relative refractive index difference Δ is expressedin terms of percentage, and the respective refractive indices ofindividual regions in the above-mentioned defining expression may bearranged in any order. Consequently, a negative value of Δ indicatesthat the refractive index of its corresponding region is lower than thatof the cladding region 120.

[0062] The optical fiber according to the first embodiment has azero-dispersion wavelength at 1.44 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.060 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.054 ps/nm²/km, andthe cutoff wavelength is 0.96 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−18.5 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −9.6ps/nm/km, the dispersion at a wavelength of 1.45 μm is 0.6 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 6.2 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 8.8 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.06 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 49.1 μm².

[0063] (Second Embodiment)

[0064]FIG. 3 is a refractive index profile of an optical fiber accordingto the second embodiment. The basic configuration of the optical fiberaccording to the second embodiment is similar to that of the firstembodiment shown in FIG. 2A, but is different therefrom in that thecladding region 120 shown in FIG. 2A is modified to have a depressedcladding structure. Referring to FIG. 2A for explanation, the opticalfiber according to the second embodiment comprises a core region 110with an outside diameter 2 a having a refractive index n₁, and acladding region 120 provided on the outer periphery of the core region110. The cladding region 120 is constituted by an inner cladding with anoutside diameter 2 b, provided in contact with the core region 110,having a refractive index n₃ (<n₁); and an outer cladding, provided onthe outer periphery of the inner cladding, having a refractive index n₂(<n₁, >n₃). Here, the outside diameter 2 a of the core region 110 is 5.2μm, whereas the outside diameter 2 b of the inner cladding region is10.9 μm. Also, with reference to the refractive index n₂ of the outercladding region, the relative refractive index difference Δ₁(=(n₁−n₂)/n₂) of the core region is 0.55%, whereas the relativerefractive index difference Δ₂ (=(n₃−n₂)/n₂) of the inner cladding is−0.05%. Such an optical fiber is obtained when, for example, whilesilica is used as a base, the core region and the inner cladding aredoped with Ge element and F element, respectively.

[0065] As for the relationship between the refractive index profile 250shown in FIG. 3 and the cross-sectional structure shown in FIG. 2A, theabscissa of the refractive index profile 250 corresponds to individualparts, along the line L in FIG. 2A, on a cross section perpendicular tothe center axis of the core region 110. Hence, in the refractive indexprofile 250 of FIG. 3, areas 251, 252, and 253 indicate the refractiveindices in individual parts on the line L of the core region 110, theinner cladding constituting the cladding region 120, and the outercladding constituting the cladding region 120, respectively.

[0066] The optical fiber according to the second embodiment has azero-dispersion wavelength at 1.46 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.053 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.049 ps/nm²/km, andthe cutoff wavelength is 0.93 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−18.5 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −10.1ps/nm/km, the dispersion at a wavelength of 1.45 μm is −0.5 ps/nm/km,the dispersion at a wavelength of 1.55 μm is 4.3 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 6.7 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.20 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 47.2 μm².

[0067] (Third Embodiment)

[0068]FIG. 4 is a refractive index profile of an optical fiber accordingto the third embodiment. The basic configuration of the optical fiberaccording to the third embodiment is also similar to that of the firstembodiment shown in FIG. 2A, but is different therefrom in that the coreregion 110 shown in FIG. 2A is constituted by a first core and a secondcore. Referring to FIG. 2A for explaining the configuration of theoptical fiber according to the third embodiment, the core region 110comprises a first core, with an outside diameter 2 a, having a maximumrefractive index n₁ at the optical axis center; and a second core withan outside diameter 2 b, provided on the outer periphery of the firstcore, having a refractive index n₂ (<n₁). The cladding region 120provided on the outer periphery of the second core has a refractiveindex n₃ (<n₂).

[0069] As for the relationship between the refractive index profile 350shown in FIG. 4 and the cross-sectional structure shown in FIG. 2A, theabscissa of the refractive index profile 350 corresponds to individualparts, along the line L in FIG. 2A, on a cross section perpendicular tothe center axis of the core region 110. Hence, in the refractive indexprofile 350 of FIG. 4, areas 351, 352, and 353 indicate the refractiveindices in individual parts on the line L of the first core constitutingthe core region 110, the second core constituting the core region 110,and the cladding region 120, respectively. Here, the outside diameter 2a of the first core constituting the core region 110 is 6.4 μm, whereasthe outside diameter 2 b of the second core region is 16.0 μm. Withreference to the refractive index n₃ of the cladding region 120, therelative refractive index difference Δ₁ (=(n₁−n₃)/n₃) of the first coreis 0.60%, whereas the relative refractive index difference Δ₂(=(n₂−n₃)/n₃) of the second core is 0.10%. Such an optical fiber isobtained when, for example, while silica is used as a base, the firstcore and the second core are doped with their respective appropriateamounts of Ge element.

[0070] The optical fiber according to the third embodiment has azero-dispersion wavelength at 1.42 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.079 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.070 ps/nm²/km, andthe cutoff wavelength is 1.19 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−20.8 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −10.6ps/nm/km, the dispersion at a wavelength of 1.45 μm is 2.1 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 9.3 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 12.8 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.006 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 63.6 μm².

[0071] (Fourth Embodiment)

[0072]FIG. 5 is a refractive index profile of an optical fiber accordingto the fourth embodiment. As in the first embodiment shown in FIG. 2A,the optical fiber according to the fourth embodiment comprises a coreregion 110 and a cladding region 120. However, it differs from theabove-mentioned third embodiment in that the cladding region 120comprises a depressed structure. Referring to FIG. 2A for explaining theconfiguration of the optical fiber according to the fourth embodiment,as in the third embodiment, the core region 110 comprises a first core,with an outside diameter 2 a, having a maximum refractive index n₁ atthe optical axis center; and a second core with an outside diameter 2 b,provided on the outer periphery of the first core, having a refractiveindex n₂ (<n₁). The cladding region 120 comprises an inner cladding withan outside diameter 2 c, provided in contact with the outer periphery ofthe second core, having a refractive index n₄ (<n₂); and an outercladding, provided on the outer periphery of the inner cladding, havinga refractive index n₃ (>n₄, <n₂).

[0073] As for the relationship between the refractive index profile 450shown in FIG. 5 and the cross-sectional structure shown in FIG. 2A, theabscissa of the refractive index profile 450 corresponds to individualparts, along the line L in FIG. 2A, on a cross section perpendicular tothe center axis of the core region 110. Hence, in the refractive indexprofile 450 of FIG. 5, areas 451, 452, 453, and 454 indicate therefractive indices in individual parts on the line L of the first coreconstituting the core region 110, the second core constituting the coreregion 110, the inner cladding constituting the cladding region 120, andthe outer cladding constituting the cladding region 120, respectively.Here, the outside diameter 2 a of the first core constituting the coreregion 110 is 6.3 μm, the outside diameter 2 b of the second core regionis 16.1 μm, and the outside diameter 2 c of the inner cladding is 28.8μm. With reference to the refractive index of pure silica, the relativerefractive index difference Δ₁ (=(n₁−n₃)/n₃) of the first core is 0.60%,the relative refractive index difference Δ₂ (=(n₂−n₃)/n₃) of the secondcore is 0.10%, and the relative refractive index difference Δ₄(=(n₄−n₃)/n₃) of the inner cladding is −0.05%. Such an optical fiber isobtained when, for example, while silica is used as a base, the firstcore and the second core are doped with their respective appropriateamounts of Ge element, whereas the inner cladding is doped with Felement.

[0074] The optical fiber according to the fourth embodiment has azero-dispersion wavelength at 1.41 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.081 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.070 ps/nm²/km, andthe cutoff wavelength is 1.15 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−20.3 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −9.9ps/nm/km, the dispersion at a wavelength of 1.45 μm is 3.1 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 10.2 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 13.7 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.004 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 62.0 μm².

[0075] (Fifth Embodiment)

[0076]FIG. 6 is a refractive index profile of an optical fiber accordingto the fifth embodiment. The basic configuration of the optical fiberaccording to the fifth embodiment is also similar to the firstembodiment shown in FIG. 2A, and is constituted by a core region 110 anda cladding region 120. As for the configuration of the optical fiberaccording to the fifth embodiment shown in FIG. 2A, the core region 110comprises a first core, with an outside diameter 2 a, extending along apredetermined axis and having a refractive index n₁; a second core withan outside diameter 2 b, provided on the outer periphery of the firstcore, having a refractive index n₂ (<n₁); and a third core with anoutside diameter 2 c, provided on the outer periphery of the secondcore, having a refractive index n₃ (>n₂, <n₁). The cladding region 120provided on the outer periphery of the third core 3 has a refractiveindex n₄ (<n₁, <n₃).

[0077] As for the relationship between the refractive index profile 550shown in FIG. 6 and the cross-sectional structure shown in FIG. 2A, theabscissa of the refractive index profile 550 corresponds to individualparts, along the line L in FIG. 2A, on a cross section perpendicular tothe center axis of the core region 110. Hence, in the refractive indexprofile 550 of FIG. 6, areas 551, 552, 553, and 554 indicate therefractive indices in individual parts on the line L of the first coreconstituting the core region 110, the second core constituting the coreregion 110, the third core constituting the core region 110, and thecladding region 120, respectively. Here, the outside diameter 2 a of thefirst core is 5.3 μm, the outside diameter 2 b of the second core regionis 10.0 μm, and the outside diameter 2 c of the third core region is16.6 μm. With reference to the refractive index of the cladding region,the relative refractive index difference Δ₁ (=(n₁−n₄)/n₄) of the firstcore is 0.58%, the relative refractive index difference of the secondcore is 0% since it is set such that n₂=n₄, and the relative refractiveindex difference Δ₃ (=(n₃−n₄)/n₄) of the third core is 0.14%. Such anoptical fiber is obtained when, for example, while silica is used as abase, the first core and the third core are doped with their respectiveappropriate amounts of Ge element.

[0078] The optical fiber according to the fifth embodiment has azero-dispersion wavelength at 1.48 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.064 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.064 ps/nm²/km, andthe cutoff wavelength is 1.24 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−20.3 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −11.9ps/nm/km, the dispersion at a wavelength of 1.45 μm is −1.9 ps/nm/km,the dispersion at a wavelength of 1.55 μm is 4.8 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 8.0 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.008 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 53.9 μm².

[0079] (Sixth Embodiment)

[0080]FIG. 7 is a refractive index profile of an optical fiber accordingto the sixth embodiment. As in the first embodiment shown in FIG. 2A,the basic configuration of the optical fiber according to the sixthembodiment comprises a core region 110 and a cladding region 120.However, it differs from the fifth embodiment in that the claddingregion 120 comprises a depressed cladding structure. Referring to FIG.2A for explaining the configuration of the optical fiber according tothe sixth embodiment, the core region 110 comprises a first core, withan outside diameter 2 a, extending along a predetermined axis and havinga refractive index n₁; a second core with an outside diameter 2 b,provided on the outer periphery of the first core, having a refractiveindex n₂ (<n₁); and a third core with an outside diameter 2 c, providedon the outer periphery of the second core, having a refractive index n₃(<n₁, >n₂). The cladding region 120 of depressed cladding structurecomprises an inner cladding with an outside diameter 2 d, provided onthe outer periphery of the third core, having a refractive index n₅(<n₃); and an outer cladding, provided on the outer periphery of theinner cladding, having a refractive index n₄ (<n₃, >n₅).

[0081] As for the relationship between the refractive index profile 650shown in FIG. 7 and the cross-sectional structure shown in FIG. 2A, theabscissa of the refractive index profile 650 corresponds to individualparts, along the line L in FIG. 2A, on a cross section perpendicular tothe center axis of the core region 110. Hence, in the refractive indexprofile 650 of FIG. 7, areas 651, 652, 653, 654, and 655 indicate therefractive indices in individual parts on the line L of the first coreconstituting the core region 110, the second core constituting the coreregion 110, the third core constituting the core region 110, the innercladding constituting the cladding region 120, and the outer claddingconstituting the cladding region 120, respectively. Here, the outsidediameter 2 a of the first core is 5.7 μm, the outside diameter 2 b ofthe second core is 16.2 μm, the outside diameter 2 c of the third coreregion is 23.0 μm, and the outside diameter 2 d of the inner cladding 2d is 34.4 μm. With reference to the refractive index of the outercladding region, the relative refractive index difference Δ₁(=(n₁−n₄)/n₄) of the first core is 0.50%, the relative refractive indexdifference of the second core is 0% since it is set such that n₂=n₄, therelative refractive index difference Δ₃ (=(n₃−n₄)/n₄) of the third coreis 0.16%, and the relative refractive index difference Δ₅ (=(n₅−n₄)/n₄)of the inner cladding is −0.10%. Such an optical fiber is obtained when,for example, while silica is used as a base, the first core and thethird core are doped with their respective appropriate amounts of Geelement, whereas the inner cladding is doped with F element.

[0082] The optical fiber according to the sixth embodiment has azero-dispersion wavelength at 1.42 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.056 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.052 ps/nm²/km, andthe cutoff wavelength is 1.23 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−16.4 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −7.9ps/nm/km, the dispersion at a wavelength of 1.45 μm is 1.6 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 6.6 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 9.2 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.02 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 57.1 μm².

[0083] (Seventh and Eighth Embodiments)

[0084]FIG. 8 is a refractive index profile of optical fibers accordingto the seventh and eighth embodiments. Both of the seventh and eighthembodiments have the same configuration, each comprising a core region110 and a cladding region 120 as with the first embodiment shown in FIG.2A. Referring to FIG. 2A for explaining the configuration of the opticalfibers according to the seventh and eighth embodiments, the core region110 comprises a first core, with an outside diameter 2 a, extendingalong a predetermined axis and having a refractive index n₁; and asecond core with an outside diameter 2 b, provided on the outerperiphery of the first core, having a refractive index n₂ (>n₁). Thecladding region 120, provided on the outer periphery of the second core,has a refractive index n₃ (<n₂).

[0085] As for the relationship between the refractive index profile 750shown in FIG. 8 and the cross-sectional structure shown in FIG. 2A, theabscissa of the refractive index profile 750 corresponds to individualparts, along the line L in FIG. 2A, on a cross section perpendicular tothe center axis of the core region 110. Hence, in the refractive indexprofile 750 of FIG. 8, areas 751, 752, and 753 indicate the refractiveindices in individual parts on the line L of the first core constitutingthe core region 110, the second core constituting the core region 110,and the cladding region 120, respectively.

[0086] In the optical fiber according to the seventh embodiment, theoutside diameter 2 a of the first core is 2.8 μm, whereas the outsidediameter 2 b of the second core is 5.6 μm. With reference to therefractive index of the cladding region, the relative refractive indexdifference Δ₁ of the first core is 0% since it is set such that n₁=n₃,whereas the relative refractive index difference Δ₂ (=(n₂−n₃)/n₃) of thesecond core is 0.7%. Such an optical fiber is obtained when, forexample, while silica is used as a base, the second core is doped withGe element.

[0087] The optical fiber according to the seventh embodiment has azero-dispersion wavelength at 1.41 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.075 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.061 ps/nm²/km, andthe cutoff wavelength is 1.10 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−20.1 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −9.3ps/nm/km, the dispersion at a wavelength of 1.45 μm is 3.0 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 9.4 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 12.4 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.3 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 67.3 μm².

[0088] In the optical fiber according to the eighth embodiment, on theother hand, the outside diameter 2 a of the first core is 3.2 μm,whereas the outside diameter 2 b of the second core is 6.4 μm. Withreference to the refractive index of the cladding region, the relativerefractive index difference Δ₁ (=(n₁−n₃)/n₃) of the first core is −0.2%,whereas the relative refractive index difference Δ₂ (=(n₂−n₃)/n₃) of thesecond core is 0.7%. Such an optical fiber is obtained when, forexample, while silica is used as a base, the first core and the secondcore are doped with F element and Ge element, respectively.

[0089] The optical fiber according to the eighth embodiment has azero-dispersion wavelength at 1.42 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.084 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.068 ps/nm²/km, andthe cutoff wavelength is 1.17 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−22.9 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −11.1ps/nm/km, the dispersion at a wavelength of 1.45 μm is 2.4 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 9.9 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 13.2 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.2 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 79.1 μm².

[0090] (Ninth and Tenth Embodiments)

[0091]FIG. 9 is a refractive index profile of optical fibers accordingto the ninth and tenth embodiments. Both of the ninth and tenthembodiments have the same configuration, each comprising a core region110 and a cladding region 120 as with the first embodiment shown in FIG.2A. However, the ninth and tenth embodiments differ from the seventh andeighth embodiments in that the cladding region 120 comprises a depressedcladding structure. Referring to FIG. 2A for explaining theconfiguration of the optical fibers according to the ninth and tenthembodiments, the core region 110 comprises a first core, with an outsidediameter 2 a, extending along a predetermined axis and having arefractive index n₁; and a second core with an outside diameter 2 b,provided on the outer periphery of the first core, having a refractiveindex n₂ (>n₁). The cladding region with the depressed claddingstructure comprises an inner cladding with an outside diameter 2 c,provided on the outer periphery of the second core, having a refractiveindex n₄ (<n₁); and an outside cladding, provided on the outer peripheryof the inner cladding, having a refractive index n₃ (>n₄).

[0092] As for the relationship between the refractive index profile 850shown in FIG. 9 and the cross-sectional structure shown in FIG. 2A, theabscissa of the refractive index profile 850 corresponds to individualparts, along the line L in FIG. 2A, on a cross section perpendicular tothe center axis of the core region 110. Hence, in the refractive indexprofile 850 of FIG. 9, areas 851, 852, 853, and 854 indicate therefractive indices in individual parts on the line L of the first coreconstituting the core region 110, the second core constituting the coreregion 110, the inner cladding constituting the cladding region 120, andthe outer cladding constituting the cladding region 120, respectively.

[0093] In the optical fiber according to the ninth embodiment, theoutside diameter 2 a of the first core is 3.8 μm, the outside diameter 2b of the second core is 7.1 μm, and the outside diameter 2 c of theinner cladding is 10.6 μm. With reference to the refractive index of theouter cladding, the relative refractive index difference Δ₁ of the firstcore is 0% since it is set such that n₁=n₃, the relative refractiveindex difference Δ₂ (=(n₂−n₃)/n₃) of the second core is 0.7%, and therelative refractive index difference Δ₄ (=(n₄−n₃)/n₃) of the innercladding is −0.2%. Such an optical fiber is obtained when, for example,while silica is used as a base, the second core and the inner claddingare doped with Ge element and F element, respectively.

[0094] The optical fiber according to the ninth embodiment has azero-dispersion wavelength at 1.42 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.077 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.061 ps/nm²/km, andthe cutoff wavelength is 1.22 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−21.6 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −10.2ps/nm/km, the dispersion at a wavelength of 1.45 μm is 2.2 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 9.1 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 12.1 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.2 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 73.5 μm².

[0095] In the optical fiber according to the tenth embodiment, on theother hand, the outside diameter 2 a of the first core is 2.6 μm, theoutside diameter 2 b of the second core is 6.4 μm, and the outsidediameter 2 c of the inner cladding is 9.6 μm. With reference to therefractive index of the outer cladding, the relative refractive indexdifference Δ₁(=(n₁−n₃)/n₃) of the first core is −0.2%, the relativerefractive index difference Δ₂ (=(n₂−n₃)/n₃) of the second core is 0.7%,and the relative refractive index difference Δ₄ (=(n₄−n₃)/n₃) of theinner cladding is −0.2%. Such an optical fiber is obtained when, forexample, while silica is used as a base, the second core is doped withGe element, whereas the first core and the inner cladding are each dopedwith F element.

[0096] The optical fiber according to the tenth embodiment has azero-dispersion wavelength at 1.44 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.070 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.058 ps/nm²/km, andthe cutoff wavelength is 1.18 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−21.5 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −10.8ps/nm/km, the dispersion at a wavelength of 1.45 μm is 0.7 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 7.3 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 10.1 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.03 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 59.6 μm².

[0097] (Eleventh and Twelfth Embodiments)

[0098]FIG. 10 is a refractive index profile of optical fibers accordingto the eleventh and twelfth embodiments. Both of the eleventh andtwelfth embodiments have the same configuration, each comprising a coreregion 110 and a cladding region 120 as with the first embodiment shownin FIG. 2A. Referring to FIG. 2A for explaining the configuration of theoptical fibers according to the eleventh and twelfth embodiments, thecore region 110 comprises a first core, with an outside diameter 2 a,extending along a predetermined axis and having a refractive index n₁; asecond core with an outside diameter 2 b, provided on the outerperiphery of the first core, having a refractive index n₂ (>n₁); a thirdcore with an outside diameter 2 c, provided on the outer periphery ofthe second core, having a refractive index n₃ (<n₂); and, a fourth corewith an outside diameter 2 d, provided on the outer periphery of thethird core, having a refractive index n₄ (<n₂, >n₃). The cladding region120 provided on the outer periphery of the fourth core has a refractiveindex n₅ (<n₄).

[0099] As for the relationship between the refractive index profile 950shown in FIG. 10 and the cross-sectional structure shown in FIG. 2A, theabscissa of the refractive index profile 950 corresponds to individualparts, along the line L in FIG. 2A, on a cross section perpendicular tothe center axis of the core region 110. Hence, in the refractive indexprofile 950 of FIG. 10, areas 951, 952, 953, 954, and 955 indicate therefractive indices in individual parts on the line L of the first coreconstituting the core region 110, the second core constituting the coreregion 110, the third core constituting the core region 110, the fourthcore constituting the core region 110, and the cladding region 120,respectively.

[0100] In the optical fiber according to the eleventh embodiment, theoutside diameter 2 a of the first core is 2.7 μm, the outside diameter 2b of the second core is 5.4 μm, the outside diameter 2 b of the thirdcore is 8.1 μm, and the outside diameter 2 d of the fourth core is 10.8μm. With reference to the refractive index of the cladding region, therelative refractive index difference Δ₁ of the first core is 0% since itis set such that n₁=n₃, the relative refractive index difference Δ₂(=(n₂−n₅)/n₅) of the second core is 0.8%, the relative refractive indexdifference Δ₃ of the third core is 0% since it is set such that n₃=n₅,and the relative refractive index difference Δ₄ (=(n₄−n₅)/n₅) of thefourth core is 0.1%. Such an optical fiber is obtained when, forexample, while silica is used as a base, the second core and the fourthcore are each doped with Ge.

[0101] The optical fiber according to the eleventh embodiment has azero-dispersion wavelength at 1.42 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.080 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.065 ps/nm²/km, andthe cutoff wavelength is 1.16 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−21.8 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −10.5ps/nm/km, the dispersion at a wavelength of 1.45 μm is 2.3 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 9.5 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 12.7 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.005 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 62.6 μm².

[0102] In the optical fiber according to the twelfth embodiment, on theother hand, the outside diameter 2 a of the first core is 3.2 μm, theoutside diameter 2 b of the second core is 7.0 μm, the outside diameter2 c of the third core is 9.0 μm, and the outside diameter 2 d of thefourth core 2 d is 12.8 μm. With reference to the refractive index ofthe cladding region, the relative refractive index difference Δ₁(=(n₁−n₅)/n₅) of the first core is −0.2%, the relative refractive indexdifference Δ₂ (=(n₂−n₅)/n₅) of the second core is 0.6%, the relativerefractive index difference Δ₃ (=(n₃−n₅)/n₅) of the third core is −0.2%,and the relative refractive index difference Δ₄ (=(n₄−n₅)/n₅) of thefourth core is 0.1%. Such an optical fiber is obtained when, forexample, while silica is used as a base, the second core and the fourthcore are each doped with Ge element, whereas the first core and thethird core are each doped with F element.

[0103] The optical fiber according to the twelfth embodiment has azero-dispersion wavelength at 1.41 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.088 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.071 ps/nm²/km, andthe cutoff wavelength is 1.22 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−22.5 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −10.6ps/nm/km, the dispersion at a wavelength of 1.45 μm is 3.4 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 11.0 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 14.5 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.4 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 92.7 μm².

[0104] (Thirteenth Embodiment)

[0105] The thirteenth embodiment of the optical fiber according to thepresent invention has a cross-sectional structure and a refractive indexprofile such as those basically shown in FIGS. 2A and 2B. However, theoptical fiber according to the thirteenth embodiment differs from thefirst embodiment in that, while silica is used as a base, the claddingregion 120 excluding the core region 110 is doped with fluorine(refractive index lowering agent), so as to generate a relativerefractive index difference between the core region 110 and the claddingregion 120.

[0106] In the optical fiber according to the thirteenth embodiment, thecore region 110 made of pure silica (with a refractive index n₀) has anoutside diameter 2 a of 5.6 μm. With reference to the refractive indexn₂ (<n₀) of the cladding region 120, the relative refractive indexdifference Δ₁ (=(n₀−n₂)/n₂) of the core region 110 is 0.53%. Also,though the core region 110 is constituted by pure silica (silica whichis not intentionally doped with impurities) in the thirteenthembodiment, it may be made of silica doped with chlorine.

[0107] The optical fiber according to the thirteenth embodiment has azero-dispersion wavelength at 1.41 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.057 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.048 ps/nm²/km, andthe cutoff wavelength is 1.04 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−15.7 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −7.2ps/nm/km, the dispersion at a wavelength of 1.45 μm is 2.2 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 7.1 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 9.4 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.04 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 52.2 μm².

[0108] In addition, the optical fiber according to the thirteenthembodiment yields a transmission loss of 0.17 dB/km at a wavelength of1.55 μm, thus being an optical fiber of a lower transmission loss ascompared with the embodiments whose core region is doped with Ge(yielding a transmission loss of about 0.20 dB/km at the wavelength of1.55 μm).

[0109] (Fourteenth Embodiment)

[0110] The fourteenth embodiment of the optical fiber according to thepresent invention has a refractive index profile similar to that of thethird embodiment shown in FIG. 4, while having an effective area ofabout 50 μm² at a wavelength of 1.55 μm. However, the profile form ofthe fourteenth embodiment differs from that of the third embodiment inthat the refractive index (n₁) of the first core is radially uniform.

[0111] As in the above-mentioned third embodiment, the optical fiberaccording to the fourteenth embodiment comprises a first core having arefractive index n₁; a second core, provided on the outer periphery ofthe first core, having a refractive index n₂ (<n₁); and a claddingregion, provided on the outer periphery of the second core, having arefractive index n₃ (<n₂).

[0112] In the optical fiber according to the fourteenth embodiment, theoutside diameter 2 a of the first core is 5.5 μm, whereas the outsidediameter 2 b of the second core is 23.7 μm. With reference to therefractive index n₃ of the cladding region, the relative refractiveindex difference Δ₁ (=(n₁−n₃)/n₃) of the first core is 0.59%, whereasthe relative refractive index difference Δ₂ (=(n₂−n₃)/n₃) of the secondcore is 0.06%.

[0113] The optical fiber according to the fourteenth embodiment has azero-dispersion wavelength at 1.41 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.065 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.055 ps/nm²/km, andthe cutoff wavelength is 1.25 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−16.8 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −7.7ps/nm/km, the dispersion at a wavelength of 1.45 μm is 2.5 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 8.5 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 11.2 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.00002 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 50.1 μm².

[0114] (Fifteenth Embodiment)

[0115] The fifteenth embodiment of the optical fiber according to thepresent invention also has a refractive index profile similar to that ofthe third embodiment shown in FIG. 4, while having a zero-dispersionwavelength in the vicinity of 1450 nm. However, the profile form of thefifteenth embodiment differs from that of the third embodiment in thatthe refractive index (n₁) of the first core is radially uniform.

[0116] As in the above-mentioned third embodiment, the optical fiberaccording to the fifteenth embodiment comprises a first core having arefractive index n₁; a second core, provided on the outer periphery ofthe first core, having a refractive index n₂ (<n₁); and a claddingregion, provided on the outer periphery of the second core, having arefractive index n₃ (<n₂). In the optical fiber according to thefifteenth embodiment, the outside diameter 2 a of the first core is 4.8μm, whereas the outside diameter 2 b of the second core is 15.1 μm. Withreference to the refractive index n₃ of the cladding region, therelative refractive index difference Δ₁ (=(n₁−n₃)/n₃) of the first coreis 0.65%, whereas the relative refractive index difference Δ₂(=(n₂−n₃)/n₃) of the second core is 0.06%.

[0117] The optical fiber according to the fifteenth embodiment has azero-dispersion wavelength at 1.46 (1.457) μm, and only this onezero-dispersion wavelength exists within a wavelength range of 1.20 μmto 1.60 μm. The dispersion slope at the zero-dispersion wavelength is0.060 ps/nm²/km, the dispersion slope at a wavelength of 1.55 μm is0.060 ps/nm²/km, and the cutoff wavelength is 1.07 μm. Also, thedispersion slope monotonously increases at least in a wavelength rangeof 1.30 μm to 1.55 μm; and, specifically, the dispersion at a wavelengthof 1.20 μm is −20.2 ps/nm/km, the dispersion at a wavelength of 1.30 μmis −11.1 ps/nm/km, the dispersion at a wavelength of 1.45 μm is −0.6ps/nm/km, the dispersion at a wavelength of 1.55 μm is 5.7 ps/nm/km, andthe dispersion at a wavelength of 1.60 μm is 8.7 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.00006 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 45.3 μm².

[0118] (Sixteenth Embodiment)

[0119] The sixteenth embodiment of the optical fiber according to thepresent invention has a refractive index profile similar to that of thefifth embodiment shown in FIG. 6, but differs therefrom in that therefractive index (n₂) of the second core is set higher than therefractive index (n₄) of the cladding region and in that the form of therefractive index profile of the first core is an α-type distribution(dome form such as one shown in FIG. 5).

[0120] As in the above-mentioned fifth embodiment, the optical fiberaccording to the sixteenth embodiment comprises a first core having amaximum refractive index n₁; a second core, provided on the outerperiphery of the first core, having a refractive index n₂ (<n₁); a thirdcore, provided on the outer periphery of the second core, having arefractive index n₃ (>n₂, <n₁); and a cladding region, provided on theouter periphery of the third core, having a refractive index n₄ (<n₃).

[0121] In the optical fiber according to the sixteenth embodiment, theoutside diameter 2 a of the first core is 6.7 μm, the outside diameter 2b of the second core is 13.4 μm, and the outside diameter 2 c of thethird core is 22.4 μm. With reference to the refractive index n₄ of thecladding region, the relative refractive index difference Δ₁(=(n₁−n₄)/n₄) of the first core is 0.60%, the relative refractive indexdifference Δ₂ (=(n₂−n₄)/n₄) of the second core is 0.05%, and therelative refractive index difference Δ₃ (=(n₃−n₄)/n₄) of the third coreis 0.11%.

[0122] The optical fiber according to the sixteenth embodiment has azero-dispersion wavelength at 1.47 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.065 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.065 ps/nm²/km, andthe cutoff wavelength is 1.37 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−21.1 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −12.1ps/nm/km, the dispersion at a wavelength of 1.45 μm is −1.3 ps/nm/km,the dispersion at a wavelength of 1.55 μm is 5.1 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 8.4 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.02 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 62.6 μm².

[0123] (Seventeenth Embodiment)

[0124] The seventeenth embodiment of the optical fiber according to thepresent invention has a refractive index profile similar to that of thethird embodiment shown in FIG. 4, while having a cutoff wavelengthlonger than its signal light wavelength.

[0125] As in the above-mentioned third embodiment, the optical fiberaccording to the seventeenth embodiment comprises a first core having arefractive index n₁; a second core, provided on the outer periphery ofthe first core, having a refractive index n₂ (<n₁); and a claddingregion, provided on the outer periphery of the second core, having arefractive index n₃ (<n₂).

[0126] In the optical fiber according to the seventeenth embodiment, theoutside diameter 2 a of the first core is 7.5 μm, whereas the outsidediameter 2 b of the second core is 29.0 μm. With reference to therefractive index n₃ of the cladding region, the relative refractiveindex difference Δ₁ (=(n₁−n₃)/n₃) of the first core is 0.61%, whereasthe relative refractive index difference Δ₂ (=(n₂−n₃)/n₃) of the secondcore is 0.10%.

[0127] The optical fiber according to the seventeenth embodiment has azero-dispersion wavelength at 1.40 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.071 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.059 ps/nm²/km, andthe cutoff wavelength is 1.78 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−17.4 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −7.7ps/nm/km, the dispersion at a wavelength of 1.45 μm is 3.5 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 9.7 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 12.6 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.00002 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 60.3 μm².

[0128] As for the optical fiber having a triple structure in which thecore region is constituted by the first to third cores as shown in FIGS.6 and 7, a plurality of embodiments having such a low dispersion slopethat the dispersion at a wavelength of 1.55 μm is 0.06 ps/nm²/km or lesswill now be explained.

[0129] (Eighteenth Embodiment)

[0130] The eighteenth embodiment of the optical fiber according to thepresent invention has a refractive index profile similar to that of thefifth embodiment shown in FIG. 6, while having a low dispersion slope.

[0131] As in the above-mentioned fifth embodiment, the optical fiberaccording to the eighteenth embodiment comprises a first core having arefractive index n₁; a second core, provided on the outer periphery ofthe first core, having a refractive index n₂ (<n₁); a third core,provided on the outer periphery of the second core, having a refractiveindex n₃ (>n₂, <n₁); and a cladding region, provided on the outerperiphery of the third core, having a refractive index n₄ (=n₂).

[0132] In the optical fiber according to the eighteenth embodiment, theoutside diameter 2 a of the first core is 5.5 μm, the outside diameter 2b of the second core is 22.8 μm, and the outside diameter 2 c of thethird core is 34.6 μm. With reference to the refractive index n₄ of thecladding region, the relative refractive index difference Δ₁(=(n₁−n₄)/n₄) of the first core is 0.48%, the relative refractive indexdifference of the second core is 0% since it is set such that n₂=n₄, andthe relative refractive index difference Δ₃ (=(n₃−n₄)/n₄) of the thirdcore is 0.12%.

[0133] The optical fiber according to the eighteenth embodiment has azero-dispersion wavelength at 1.41 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.058 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.040 ps/nm²/km, andthe cutoff wavelength is 1.75 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−16.5 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −7.5ps/nm/km, the dispersion at a wavelength of 1.45 μm is 2.1 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 6.8 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 8.6 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.2 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 57.1 μm².

[0134] (Nineteenth Embodiment)

[0135] The nineteenth embodiment of the optical fiber according to thepresent invention is also an optical fiber having a refractive indexprofile similar to that of the fifth embodiment shown in FIG. 6, whilehaving a low dispersion slope. The refractive index profile of thenineteenth embodiment differs from that of the fifth embodiment or thatof the above-mentioned eighteenth embodiment in that the refractiveindex (n₂) of the second core is set higher than the refractive index(n₄) of the cladding region.

[0136] As in the above-mentioned fifth embodiment, the optical fiberaccording to the nineteenth embodiment comprises a first core having arefractive index n₁; a second core, provided on the outer periphery ofthe first core, having a refractive index n₂ (<n₁); a third core,provided on the outer periphery of the second core, having a refractiveindex n₃ (>n₂, <n₁); and a cladding region, provided on the outerperiphery of the third core, having a refractive index n₄ (<n₃).

[0137] In the optical fiber according to the nineteenth embodiment, theoutside diameter 2 a of the first core is 6.2 μm, the outside diameter 2b of the second core is 19.9 μm, and the outside diameter 2 c of thethird core is 28.4 μm. With reference to the refractive index n₄ of thecladding region, the relative refractive index difference Δ₁(=(n₁−n₄)/n₄) of the first core is 0.44%, the relative refractive indexdifference Δ₂ (=(n₂−n₄)/n₄) of the second core is 0.01%, and therelative refractive index difference Δ₃ (=(n₃−n₄)/n₄) of the third coreis 0.13%.

[0138] The optical fiber according to the nineteenth embodiment has azero-dispersion wavelength at 1.38 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.065 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.047 ps/nm²/km, andthe cutoff wavelength is 1.52 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−14.5 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −5.4ps/nm/km, the dispersion at a wavelength of 1.45 μm is 4.4 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 9.4 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 11.7 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.07 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 64.5 μm².

[0139] (Twentieth Embodiment)

[0140] As with the sixth embodiment shown in FIG. 7, the twentiethembodiment of the optical fiber according to the present invention is anoptical fiber having a refractive index profile of a depressed claddingstructure, while having a low dispersion slope. In the refractive indexprofile of the twentieth embodiment, as in the above-mentionednineteenth embodiment, the refractive index (n₂) of the second core isset higher than the refractive index (n₄) of the cladding region.

[0141] In the optical fiber according to the twentieth embodiment, as inthe above-mentioned sixth embodiment, the core region comprises a firstcore having a refractive index n₁; a second core, provided on the outerperiphery of the first core, having a refractive index n₂ (<n₁); and athird core, provided on the outer periphery of the second core, having arefractive index n₃ (>n₂, <n₁). Also, the cladding region comprises aninner cladding, provided on the outer periphery of the third core,having a refractive index n₅ (<n₃); and an outer cladding, provided onthe outer periphery of the inner cladding, having a refractive index n₄(<n₃, >n₅); whereas the inner and outer claddings constitute thedepressed cladding structure.

[0142] In the optical fiber according to the twentieth embodiment, theoutside diameter 2 a of the first core is 5.6 μm, the outside diameter 2b of the second core is 19.7 μm, the outside diameter 2 c of the thirdcore is 28.1 μm, m, and the outside diameter 2 d of the inner claddingis 42.0 μm. With reference to the refractive index n₄ of the outercladding, the relative refractive index difference Δ₁ (=(n₁−n₄)/n₄) ofthe first core is 0.55%,the relative refractive index difference Δ₂(=(n₂−n₄)/n₄) of the second core is 0.01%, the relative refractive indexdifference Δ₃ (=(n₃−n₄)/n₄) of the third core is 0.16%, and the relativerefractive index difference Δ₅ (=(n₅−n₄)/n₄) of the inner cladding is−0.05%.

[0143] The optical fiber according to the twentieth embodiment has azero-dispersion wavelength at 1.40 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.059 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.043 ps/nm²/km, andthe cutoff wavelength is 1.59 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−15.8 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −6.9ps/nm/km, the dispersion at a wavelength of 1.45 μm is 2.7 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 7.4 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 9.5 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.001 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 50.4 μm².

[0144] (Twenty-first Embodiment)

[0145] The twenty-first embodiment of the optical fiber according to thepresent invention is an optical fiber having a refractive index profilesimilar to that of the above-mentioned fifth embodiment shown in FIG. 6,while having a low dispersion slope.

[0146] As in the above-mentioned fifth embodiment, the optical fiberaccording to the twenty-first embodiment comprises a first core having arefractive index n₁; a second core, provided on the outer periphery ofthe first core, having a refractive index n₂ (<n₁); a third core,provided on the outer periphery of the second core, having a refractiveindex n₃ (>n₂, <n₁); and a cladding region, provided on the outerperiphery of the third core, having a refractive index n₄ (=n₂).

[0147] In the optical fiber according to the twenty-first embodiment,the outside diameter 2 a of the first core is 6.1 μm, the outsidediameter 2 b of the second core is 17.8 μm, and the outside diameter 2 cof the third core is 25.4 μm. With reference to the refractive index n₄of the cladding region, the relative refractive index difference Δ₁(=(n₁−n₄)/n₄) of the first core is 0.45%, the relative refractive indexdifference of the second core is 0% since it is set such that n₂=n₄, andthe relative refractive index difference Δ₃ (=(n₃−n₄)/n₄) of the thirdcore is 0.14%.

[0148] The optical fiber according to the twenty-first embodiment has azero-dispersion wavelength at 1.40 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.057 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.046 ps/nm²/km, andthe cutoff wavelength is 1.44 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−15.2 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −6.5ps/nm/km, the dispersion at a wavelength of 1.45 μm is 2.7 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 7.5 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 9.8 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.1 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 64.4 μm².

[0149] (Twenty-second Embodiment)

[0150] As in the sixth embodiment shown in FIG. 7, the twenty-secondembodiment of the optical fiber according to the present invention has arefractive index profile of a depressed cladding structure, while havinga low dispersion slope. In the refractive index profile of thetwenty-second embodiment, contrary to the above-mentioned twentiethembodiment, the refractive index (n₂) of the second core is set lowerthan the refractive index (n₄) of the cladding region.

[0151] In the optical fiber according to the twenty-second embodiment,as in the above-mentioned sixth embodiment, the core region comprises afirst core having a refractive index n₁; a second core, provided on theouter periphery of the first core, having a refractive index n₂ (<n₁);and a third core, provided on the outer periphery of the second core,having a refractive index n₃ (>n₂, <n₁). Also, the cladding regioncomprises an inner cladding, provided on the outer periphery of thethird core, having a refractive index n₅ (<n₃); and an outer cladding,provided on the outer periphery of the inner cladding, having arefractive index n₄ (<n₃, >n₅); whereas the inner and outer claddingsconstitute the depressed cladding structure.

[0152] In the optical fiber according to the twenty-second embodiment,the outside diameter 2 a of the first core is 6.0 μm, the outsidediameter 2 b of the second core is 19.7 μm, the outside diameter 2 c ofthe third core is 30.0 μm, and the outside diameter 2 d of the innercladding is 44.8 μm. With reference to the refractive index n₄ of theouter cladding, the relative refractive index difference Δ₁(=(n₁−n₄)/n₄) of the first core is 0.46%, the relative refractive indexdifference Δ₂ (=(n₂−n₄)/n₄) of the second core is −0.05%, the relativerefractive index difference Δ₃ (=(n₃−n₄)/n₄) of the third core is 0.16%,and the relative refractive index difference Δ₅ (=(n₅−n₄)/n₄) of theinner cladding is −0.05%.

[0153] The optical fiber according to the twenty-second embodiment has azero-dispersion wavelength at 1.39 μm, and only this one zero-dispersionwavelength exists within a wavelength range of 1.20 μm to 1.60 μm. Thedispersion slope at the zero-dispersion wavelength is 0.052 ps/nm²/km,the dispersion slope at a wavelength of 1.55 μm is 0.023 ps/nm²/km, andthe cutoff wavelength is 1.66 μm. Also, the dispersion slopemonotonously increases at least in a wavelength range of 1.30 μm to 1.55μm; and, specifically, the dispersion at a wavelength of 1.20 μm is−14.4 ps/nm/km, the dispersion at a wavelength of 1.30 μm is −5.7ps/nm/km, the dispersion at a wavelength of 1.45 μm is 2.8 ps/nm/km, thedispersion at a wavelength of 1.55 μm is 5.9 ps/nm/km, and thedispersion at a wavelength of 1.60 μm is 7.0 ps/nm/km. Further, thebending loss at a wavelength of 1.55 μm when wound at a diameter of 32mm is 0.3 dB per turn, whereas the effective area A_(eff) at thewavelength of 1.55 μm is 55.6 μm².

[0154]FIG. 11 is a table listing various characteristics of respectiveoptical fibers according to the above-mentioned first to thirteenthembodiments. Also, FIG. 12 is a table listing various characteristics ofrespective optical fibers according to the above-mentioned fourteenth totwenty-second embodiments. As shown in these tables, each of the opticalfibers according to the first to twenty-second embodiments has only onezero-dispersion wavelength within a wavelength range of 1.20 μm to 1.60μm, whereas this zero-dispersion wavelength lies within a wavelengthrange of 1.37 μm to 1.50 μm. In particular, the zero-dispersionwavelength lies within a wavelength range of 1.37 μm to 1.43 μm in thethird, fourth, sixth to ninth, eleventh to fourteenth, and seventeenthto twenty-second embodiments, whereas it lies within a wavelength rangeof longer than 1.45 μm but not longer than 1.55 μm in the second, fifth,fifteenth, and sixteenth embodiments. In each of the embodiments, theabsolute value of dispersion slope at the zero-dispersion wavelength is0.10 ps/nm²/km or less, whereas the cutoff wavelength is 1.3 μm orshorter. Therefore, each of these optical fibers is of a single modehaving no zero-dispersion wavelength at the 1.3-μm wavelength band nor1.5-μm wavelength band, while the dispersion at each of these wavelengthbands is kept low, thereby being suitable for optical communicationsutilizing a plurality of wavelength bands. At a wavelength of 1.55 μm,the first, second, sixth, thirteenth to fifteenth, and eighteenth totwenty-second embodiments have a dispersion slope of 0.06 ps/nm²/km,with the eighteenth to twenty-second embodiments having a further lowerdispersion slope in particular.

[0155] Also, in each of the optical fibers according to the first totwenty-second embodiments, the dispersion slope monotonously changes ina wavelength range of 1.30 μm to 1.55 μm, whereas the absolution valueof dispersion at wavelengths of 1.3 μm and 1.55 μm is 12 ps/nm/km orless. Therefore, the absolute value of dispersion in the 1.3-μmwavelength band and 1.55-μm wavelength band in each of these opticalfibers is sufficiently smaller than the dispersion value (about 17ps/nm/km) in the 1.55-μm wavelength band of a conventional standardsingle-mode optical fiber having a zero-dispersion wavelength in the1.3-μm wavelength band. If the dispersion value up to that (about 17ps/nm/km) in the 1.55-μm wavelength band of the above-mentioned standardsingle-mode optical fiber is permissible in an optical transmissionsystem as a whole, then each of the respective optical fibers accordingto the first to twenty-second embodiments is suitably utilized inoptical communications having a signal light wavelength band within arange of 1.2 μm to 1.7 μm.

[0156] Further, each of the optical fibers according to the first totwenty-second embodiments has a bending loss of 0.5 dB or less per turnat a wavelength of 1.55 μm when wound at a diameter of 32 mm, with thisbending loss being 0.06 dB or less in the first to sixth, eleventh,thirteenth to seventeenth, nineteenth, and twentieth embodiments inparticular, and thus is preferable in that it can effectively suppressthe increase in loss caused by cabling and the like. Also, each of theoptical fibers according to the first to twenty-second embodiments hasan effective area A_(eff) of 45 μm² or more at a wavelength of 1.55 μm,with the effective area A_(eff) in the first, third to fourteenth, andsixteenth to twenty-second embodiments exceeding 49 μm² in particular,which is on a par with or greater than the effective area ofconventional dispersion-shifted optical fibers. Hence, the lightpropagating through the optical fiber has a lower intensity per unitcross-sectional area, whereby nonlinear optical phenomena such asfour-wave mixing can effectively be suppressed.

[0157] In the refractive index profiles 150 to 950 shown in FIGS. 2B,and 3 to 10, the maximum and minimum values of relative refractive indexdifference with reference to the refractive index of the referenceregion (the cladding region 120, or the outer cladding if the claddingregion 120 has a depressed cladding structure) of pure silica (silicawhich is not intentionally doped with impurities) is 1% or less and−0.5% or more, respectively, except for the above-mentioned thirteenthembodiment. Though the thirteenth embodiment comprises a structure inwhich the cladding region 120 is doped with fluorine so as to relativelyenhance the difference in refractive index between the core region madeof pure silica and the cladding region, the maximum value of relativerefractive index difference of the core region 110 with respect to thecladding region 120 is 1% or less even in this embodiment. While a highrefractive index region is realized by doping with Ge element, forexample; since its relative refractive index difference is 1% or less,the making of this optical fiber (refractive index control by dopingwith impurities) is relatively easy, and its transmission loss becomessmaller. While a low refractive index region, on the other hand, isrealized by doping with F element, for example; since its relativerefractive index difference is −0.5% or more, the making of this opticalfiber is easy in this regard as well.

[0158]FIG. 13 is a graph showing a dispersion characteristic of theoptical fiber according to the first embodiment with respect towavelength. As shown in this graph, the dispersion slope monotonouslyincreases in a wavelength range of 1.30 μm to 1.55 μm. Also, FIGS. 14and 15 are graphs showing transmission loss characteristics with respectto wavelength of the optical fiber according to the first embodiment incases where dehydration processing is insufficient and sufficient,respectively. As shown in these graphs, an increase in transmission losscaused by OH absorption is seen at a wavelength of 1.38 μm. In anoptical fiber having such a transmission loss characteristic as thatshown in FIG. 14, the dehydration processing is not sufficientlyeffected, so that the OH group content is large, whereby the amount ofincrease in transmission loss caused by OH absorption is about 0.5dB/km. An optical fiber having such a transmission loss characteristicsas that shown in FIG. 15, on the other hand, the dehydration processingis sufficiently effected so as to reduce the OH group content, wherebythe increase in transmission loss caused by OH absorption is suppressedto about 0.01 dB/km. When the above-mentioned wavelength band isutilized as a signal wavelength band, the zero-dispersion wavelength canbe set within a range of longer than 1.45 μm but not longer than 1.55μm. The same holds true for the respective dispersion characteristicsand transmission characteristics with respect to wavelength of theoptical fibers according to the second to twelfth and fourteenth totwenty-second embodiments.

[0159] Also, FIG. 16 is a graph showing a transmission losscharacteristic with respect to wavelength of the optical fiber accordingto the thirteenth embodiment in the case where the dehydrationprocessing is insufficient. In the thirteenth embodiment, the increasein transmission loss caused by OH absorption (at a wavelength of 1.38μm) is 0.3 dB/km when the dehydration processing is not sufficientlyeffected. If the dehydration processing is sufficiently effected,however, then the increase in transmission loss at a wavelength of 1.3μm (at a wavelength of 1.38 μm) can be suppressed to 0.01 dB/km or less,as shown in FIG. 14, also in the case of the thirteenth embodiment.

[0160] Without being restricted to the above-mentioned individualembodiments, the optical fiber according to the present invention can bemodified in various manners; and, for example, other designs arepossible within the scope of the present invention.

[0161]FIG. 17A is a graph showing relationships between effective areaA_(eff) and dispersion slope at a wavelength of 1.55 μm mainlyconcerning the eighteenth to twenty-second embodiments. In this graph,P1, P5, P7, P9, P10, and P18 to P22 are points indicating therelationships between effective area A_(eff) and dispersion slope in thefirst, fifth, seventh, ninth, tenth, and eighteenth to twenty-secondembodiments, respectively.

[0162] As can also be seen from this graph, the dispersion slope at awavelength of 1.55 μm can particularly be lowered in the case of opticalfibers (eighteenth to twenty-second embodiments) having such arefractive index profile as that shown in FIG. 6. Also, the effectivearea A_(eff) at a wavelength of 1.55 μm in the optical fibers accordingto the eighteenth to twenty-second embodiments is greater than 49 μm².

[0163] Further, FIG. 17B is a graph showing relationships between cutoffwavelength λc and bending loss per turn when bent at a diameter of 32 mmat a wavelength of 1.55 μm concerning main embodiments. In this graph,P1, P3, P4, P6, P7, P10, and P14 to P16 show the relationships betweencutoff wavelength λc and bending loss in the first, third, fourth,sixth, seventh, tenth, and fourteenth to sixteenth embodiments,respectively. Also, the hatched portion in this graph is an area inwhich points indicating relationships between cutoff wavelength λc andbending loss are intensively plotted with regard to conventional opticalfibers having a refractive index profile similar to that shown in FIG.6. Therefore, for avoiding this area (hatched portion), i.e., foryielding a bending loss of 1.0 dB/turn or less, preferably 0.06 dB/turnor less at 32 mm at a wavelength of 1.55 μm, it is preferred that thecutoff wavelength λc be 1.05 μm or more, more preferably 1.3 μm or more.

[0164] Embodiments of the optical transmission system according to thepresent invention will now be explained. FIG. 18A is a view showing aschematic configuration of an embodiment of the optical transmissionsystem according to the present invention. The optical transmissionsystem shown in this drawing comprises transmitters 11, 12; opticaltransmission lines 21, 22; a multiplexer 30; an optical fiber 40; ademultiplexer 50; optical transmission lines 61, 62; and receivers 71,72.

[0165] The transmitter 11 outputs signal light (first signal light) inthe 1.3-μm wavelength band; whereas the optical transmission line 21 isa transmission medium for guiding the signal light in the 1.3-μmwavelength band outputted from the transmitter 11 to the multiplexer 30and, for example, is a standard single-mode optical fiber having azero-dispersion wavelength in the 1.3-μm wavelength band. Thetransmitter 12 outputs signal light (second signal light) in the 1.55-μmwavelength band; whereas the optical transmission line 22 is atransmission medium for guiding the signal light in the 1.55-μmwavelength band outputted from the transmitter 12 to the multiplexer 30and, for example, is a dispersion-shifted optical fiber having azero-dispersion wavelength in the 1.55-μm wavelength band.

[0166] The multiplexer 30 multiplexes the signal light in the 1.3-μmwavelength band and signal light in the 1.55-μm wavelength bandpropagated through the optical transmission lines 21, 22, and outputsthus multiplexed light to the optical fiber 40. The optical fiber 40transmits the signal light in the 1.3-μm wavelength band and signallight in the 1.55-μm wavelength band multiplexed by the multiplexer 30toward the demultiplexer 50. The demultiplexer 50 demultiplexes thesignal light in the 1.3-μm wavelength band and signal light in the1.55-μm wavelength band propagated through the optical fiber 40.

[0167] The above-mentioned optical fiber 40 is an optical fiberaccording to the present invention having a configuration mentionedabove, in which only one zero-dispersion wavelength exists within awavelength range of 1.20 μm to 1.60 μm, whereas this zero-dispersionwavelength lies within a wavelength range of 1.37 μm to 1.50 μm(preferably within a wavelength range of 1.37 μm to 1.43 μm or within awavelength range of longer than 1.45 μm but not longer than 1.50 μm).Also, in the optical fiber 40, the absolute value of dispersion slope atthe zero-dispersion wavelength is 0.10 ps/nm²/km or less (preferably0.06 ps/nm²/km or less at a wavelength of 1.55 μm). In a more preferredembodiment, the optical fiber 40 has a dispersion slope monotonouslychanging in a wavelength range of 1.30 μm to 1.55 μm, whereas each ofthe absolute values of dispersion at wavelengths of 1.3 μm and 1.55 μmis 12 ps/nm/km or less, the bending loss at a wavelength of 1.55 μm whenwound at a diameter of 32 mm is 0.5 dB or less (preferably 0.06 dB orless) per turn, the effective area. A_(eff) at the wavelength of 1.55 μmm is 45 μm² or more (greater than 49 μm²), or the increase intransmission loss caused by OH absorption at a wavelength of 1.38 μm is0.1 dB/km or less.

[0168] The optical transmission line 61 is a transmission medium forguiding the signal light in the 1.3-μm wavelength band demultiplexed bythe demultiplexer 50 to the receiver 71 and, for example, is a standardsingle-mode optical fiber having a zero-dispersion wavelength in the1.3-μm wavelength band. The receiver 71 receives the signal light in the1.3-μm wavelength band propagated through the optical transmission line61. On the other hand, the optical transmission line 62 is atransmission medium for guiding the signal light in the 1.55-μmwavelength band demultiplexed by the demultiplexer 50 to the receiver 72and, for example, is a dispersion-shifted optical fiber having azero-dispersion wavelength in the 1.55-μm wavelength band. The receiver72 receives the signal light in the 1.55-μm wavelength band propagatedthrough the optical transmission line 62.

[0169] In the optical transmission system according to this embodiment,the signal light in the 1.3-μm wavelength band having arrived at themultiplexer 30 by way of the optical transmission line 21 after beingoutputted from the transmitter 11, and the signal light in the 1.55-μmwavelength band having arrived at the multiplexer 30 by way of theoptical transmission line 22 after being outputted from the transmitter12 are multiplexed by the multiplexer 30, and thus multiplexed lightpropagates through the optical fiber 40 and reaches the demultiplexer50. The multiplexed light having arrived at the demultiplexer 50 isdemultiplexed thereby into the signal light in the 1.3-μm wavelengthband and the signal light in the 1.55-μm wavelength band. Thedemultiplexed signal light in the 1.3-μm wavelength band reaches thereceiver 71 by way of the optical transmission line 61, whereas thesignal light in the 1.55-μm wavelength band reaches the receiver 72 byway of the optical transmission line 62.

[0170] Thus, the optical fiber 40 used in the optical transmissionsystem of this embodiment comprises a structure which realizes favorableoptical communications in both of the 1.3-μm wavelength band and 1.55-μmwavelength band, whereby the optical transmission system employing theoptical fiber 40 enables large-capacity communications.

[0171] Without being restricted to the above-mentioned embodiment, theoptical fiber according to the present invention can be modified invarious manners. For example, the optical fiber, which is a transmissionmedium, disposed between the multiplexer 30 and demultiplexer 50 may beconstituted by a plurality of optical fibers 40 a to 40 c as shown inFIG. 18B.

[0172] According to the present invention, as explained in theforegoing, the optical fiber has only one zero-dispersion wavelengthwithin a wavelength range of 1.37 μm to 1.50 μm including a wavelengthof 1.38 μm at which an increase in transmission loss caused by OHabsorption is seen, preferably within a wavelength range of 1.37 μm to1.43 μm, or within a wavelength range of longer than 1.45 μm but notlonger than 1.50 μm, whereas no zero-dispersion wavelength exists in thevicinity of the 1.3-μm wavelength band and 1.55-μm wavelength bandsandwiching these wavelength ranges. Therefore, when these wavelengthbands are utilized as a signal light wavelength band, dispersion isintentionally generated, so as to effectively suppress nonlinear opticalphenomen a such as four-wave mixing. Also, since the absolute value ofdispersion slope at thus set zero-dispersion wavelength is 0.10ps/nm²/km or less (preferably 0.06 ps/nm²/km or less at a wavelength of1.55 μm), the respective dispersions in the 1.3-μm wavelength band and1.55-μm wavelength band are homogenized. When such an optical fiber isemployed in the transmission line of the optical transmission system,favorable optical communications are possible in both of the 1.3-μmwavelength band and 1.55-μm wavelength band.

[0173] From the invention thus described, it will be obvious that theembodiments of the invention may be varied in many ways. Such variationsare not to be regarded as a departure from the spirit and scope of theinvention, and all such modifications as would be obvious to one skilledin the art are intended for inclusion within the scope of the followingclaims.

What is claimed is:
 1. An optical fiber having only one zero-dispersion wavelength within a wavelength range of 1.20 μm to 1.60 μm, said zero-dispersion wavelength existing within a wavelength range of 1.37 μm to 1.50 μm, said optical fiber having a positive dispersion slope at said zero-dispersion wavelength and an effective area of greater than 60 μm² at a wavelength of 1.55 μm.
 2. An optical fiber according to claim 1 , wherein said dispersion slope has an absolute value of 0.10 ps/nm²/km or less.
 3. An optical fiber according to claim 1 , wherein said optical fiber has a dispersion slope of 0.06 ps/nm²/km or less at the wavelength of 1.55 μm.
 4. An optical fiber according to claim 1 , wherein an absolute value of dispersion at the wavelength of 1.55 μm is 12 ps/nm/km or less.
 5. An optical fiber according to claim 1 , wherein the absolute value of dispersion at the wavelength of 1.55 μm m is 6 ps/nm/km or more.
 6. An optical fiber according to claim 5 , wherein an absolute value of dispersion at a wavelength of 1.30 μm is 6 ps/nm/km or more but 12 ps/nm/km or less.
 7. An optical fiber according to claim 1 , wherein said optical fiber has a bending loss which becomes 0.5 dB/turn or less at a wavelength of 1.55 μm when wound at a diameter of 32 mm.
 8. An optical fiber according to claim 7 , wherein said optical fiber has a bending loss which becomes 0.06 dB/turn or less at a wavelength of 1.55 μm when wound at a diameter of 32 mm.
 9. An optical fiber according to claim 1 , wherein said optical fiber has a cutoff wavelength of 1.05 μm or more.
 10. An optical fiber according to claim 9 , wherein said optical fiber has a cutoff wavelength of 1.30 μm or more.
 11. An optical fiber according to claim 1 , wherein an increase in transmission loss caused by OH absorption at a wavelength of 1.38 μm is 0.1 dB/km or less.
 12. An optical fiber according to claim 1 , wherein said optical fiber has a refractive index profile in which maximum and minimum values of relative refractive index difference with reference to a refractive index of pure silica are 1% or less and −0.5% or more, respectively.
 13. An optical fiber according to claim 1 , wherein said optical fiber comprises: a core region extending along a predetermined axis and having a predetermined refractive index; and a cladding region provided on the outer periphery of said core region.
 14. An optical fiber according to claim 13 , wherein said cladding region comprises an inner cladding, in contact with the outer periphery of said core region, having a lower refractive index than said core region; and, an outer cladding, provided on the outer periphery of said inner cladding, having a refractive index higher than that of said inner cladding and lower than that of said core region.
 15. An optical fiber according to claim 1 , wherein said optical fiber comprises: a core region which is a region extending along a predetermined axis, and comprising a first core having a predetermined refractive index, and a second core, provided on the outer periphery of said first core, having a lower refractive index than said first core; and a cladding region provided on the outer periphery of said core region.
 16. An optical fiber according to claim 15 , wherein said cladding region comprises an inner cladding, in contact with the outer periphery of said second core, having a lower refractive index than said second core; and, an outer cladding, provided on the outer periphery of said inner cladding, having a refractive index higher than that of said inner cladding and lower than that of said second core.
 17. An optical fiber according to claim 1 , wherein said optical fiber comprises: a core region which is a region extending along a predetermined axis, and comprising a first core having a predetermined refractive index, a second core, provided on the outer periphery of said first core, having a lower refractive index than said first core, and a third core, provided on the outer periphery of said second core, having a higher refractive index than said second core; and a cladding region provided on the outer periphery of said core region.
 18. An optical fiber according to claim 17 , wherein said second core has a refractive index equal to or higher than that of said cladding region.
 19. An optical fiber according to claim 17 , wherein said cladding region comprises an inner cladding, in contact with the outer periphery of said third core, having a lower refractive index than said third core; and, an outer cladding, provided on the outer periphery of said inner cladding, having a refractive index higher than that of said inner cladding and lower than that of said third core.
 20. An optical fiber according to claim 19 , wherein said second core has a refractive index equal to or higher than that of said outer cladding.
 21. An optical fiber according to claim 1 , wherein said optical fiber comprises: a core region which is a region extending along a predetermined axis, and comprising a first core having a predetermined refractive index, and a second core, provided on the outer periphery of said first core, having a higher refractive index than said first core; and a cladding region provided on the outer periphery of said core region.
 22. An optical fiber according to claim 21 , wherein said cladding region comprises an inner cladding, in contact with the outer periphery of said second core, having a lower refractive index than said second core; and, an outer cladding, provided on the outer periphery of said inner cladding, having a refractive index higher than that of said inner cladding and lower than that of said second core.
 23. An optical fiber according to claim 1 , wherein said optical fiber comprises: a core region which is a region extending along a predetermined axis, and comprising a first core having a predetermined refractive index, a second core, provided on the outer periphery of said first core, having a higher refractive index than said first core, a third core, provided on the outer periphery of said second core, having a lower refractive index than said second core, and a fourth core, provided on the outer periphery of said third core, having a higher refractive index than said third core; and a cladding, provided on the outer periphery of said core region, having a lower refractive index than said fourth core.
 24. An optical fiber having only one zero-dispersion wavelength within a wavelength range of 1.20 μm to 1.60 μm, said zero-dispersion wavelength existing within a wavelength range of 1.37 μm to 1.43 μm, said optical fiber having a positive dispersion slope at said zero-dispersion wavelength and an effective area of greater than 50 μm² at a wavelength of 1.55 μm.
 25. An optical fiber according to claim 24 , wherein said effective area is greater than 60 μm² at the wavelength of 1.55 μm.
 26. An optical fiber according to claim 24 , wherein said dispersion slope has an absolute value of 0.10 ps/nm²/km or less.
 27. An optical fiber according to claim 24 , wherein said optical fiber has a dispersion slope of 0.06 ps/nm²/km or less at a wavelength of 1.55 μm.
 28. An optical fiber according to claim 24 , wherein an absolute value of dispersion at the wavelength of 1.55 μm is 12 ps/nm/km or less.
 29. An optical fiber according to claim 24 , wherein the absolute value of dispersion at the wavelength of 1.55 μm is 6 ps/nm/km or more.
 30. An optical fiber according to claim 24 , wherein an absolute value of dispersion at a wavelength of 1.30 μm m is 6 ps/nm/km or more but 12 ps/nm/km or less.
 31. An optical fiber according to claim 24 , wherein said optical fiber has a bending loss which becomes 0.5 dB/turn or less at a wavelength of 1.55 μm when wound at a diameter of 32 mm.
 32. An optical fiber according to claim 31 , wherein said optical fiber has a bending loss which becomes 0.06 dB/turn or less at a wavelength of 1.55 μm when wound at a diameter of 32 mm.
 33. An optical fiber according to claim 24 , wherein said optical fiber has a cutoff wavelength of 1.05 μm or more.
 34. An optical fiber according to claim 33 , wherein said optical fiber has a cutoff wavelength of 1.30 μm or more.
 35. An optical fiber according to claim 24 , wherein an increase in transmission loss caused by OH absorption at a wavelength of 1.38 μm is 0.1 dB/km or less.
 36. An optical fiber having only one zero-dispersion wavelength within a wavelength range of 1.20 μm to 1.60 μm, said zero-dispersion wavelength existing within a wavelength range of longer than 1.45 μm but not longer than 1.50 μm, said optical fiber having a positive dispersion slope at said zero-dispersion wavelength.
 37. An optical fiber having only one zero-dispersion wavelength within a wavelength range of 1.20 μm to 1.60 μm, said zero-dispersion wavelength existing within a wavelength range of 1.37 μm to 1.50 μm, said optical fiber having a positive dispersion slope at said zero-dispersion wavelength and a bending loss which becomes 0.06 dB/turn or less at a wavelength of 1.55 μm when wound at a diameter of 32 mm.
 38. An optical fiber having only one zero-dispersion wavelength within a wavelength range of 1.20 μm to 1.60 μm, said zero-dispersion wavelength existing within a wavelength range of 1.37 μm to 1.50 μm, said optical fiber having a positive dispersion slope at said zero-dispersion wavelength and a cutoff wavelength of 1.05 μm or more.
 39. An optical fiber according to claim 38 , wherein said cutoff wavelength is 1.30 μm or more. 